This invention relates to a tin coated steel stock material having excellent formability and good corrosion resistance, a method of producing such stock material and a method of producing seamless steel containers from such stock through a drawing and ironing operation.
Various processes are presently being utilized for forming seamless containers from flat blanks. One well-known, common commercial procedure involves first drawing a circular blank into a cup form by forcing the blank through a drawing die by means of a punch mounted on a press. Subsequently, the cup goes through an ironing operation wherein the cup is passed through one or more ironing dies whose inside diameters are progressively slightly smaller than the outside diameter of the cup to form a container body.
In the ironing operation, the sidewall of the cup is elongated by reducing its thickness without reducing the inside diameter of the cup. The ironed body is trimmed to a constant length and then necked in and flanged at its open end. After the container body is filled with product, a closure end is double seamed to the flange to seal the container and its contents. For greater detail, reference is made to an article appearing in the November, 1973 issue of AEROSOL AGE magazine entitled "The drawn and Ironed Can--Understanding the Technology".
Heretofore, two types of tin coated stock material have been used in the commercial production of seamless tinplated containers. One such type is commonly referred to as matte finish and the other as bright finish.
The production of matte finish can stock is well known and that portion of the process which is relevant hereto conventionally involves temper rolling of the steel base to provide a cold reduction of up to approximately 3% in thickness to control the temper of steel base, electroplating a tin coating onto both sides of the steel base, for example, a coating of 0.25 lb/B.B. per side, and drying the tinplated stock normally in the range of 130.degree. F. to 180.degree. F. to remove moisture. While both matte and bright tinplate provide increased formability when compared to steel stock, it is stated in U.S. Pat. No. 3,360,157 that the matte tinplate has an advantage over bright tinplate because it provides better formability. This apparently reduces the problem of fracturing of the metal as it is formed into a can or during later necking and flanging.
Bright finish stock plate is formed generally in the same manner as matte finish stock with the added step of heating the tinplated steel above its melting point to reflow the tin layer and to form an iron-tin alloy layer giving the stock a bright appearance. The melting point of tin is approximately 449.degree. F. to 450.degree. F. depending on the purity of the tin. The reflux heating temperature used by commercial manufactures is generally in the range of 450.degree. F. to 455.degree. F. The alloy layer increases the corrosion resistance of the metal by providing a further protective layer covering the steel base.
While the use of bright stock increases the corrosion resistance of the finished container, bright stock does not exhibit as great an amount of formability as the matte stock. This is believed to occur because the iron-tin alloy layer of the bright stock is of a harder and more brittle nature than the free or non-alloy tin layer, thereby resulting in increased friction between the steel substrate and the dies during the forming and flanging steps.
For beer and beverage containers, the major portion of drawn and ironed container production, matte finish has been widely used because of its superior formability over bright finish stock. While the corrosion resistance is not as good as bright finish, it has been found to be generally adequate for beer and beverage drinks, if organic coatings are later applied and cured.
For sanitary containers used in packaging such products as food, corrosion resistance is of the utmost importance and therefore bright finish can stock is used almost exclusively despite its poorer formability. Due to the poorer formability of bright stock, sanitary containers generally are not commercially formed by drawing and ironing. Instead, the slower and less economical three-piece container manufacturing method is employed. In conventional three-piece production methods, a flat sheet or blank of stock is rolled around a mandrel with the abutting lateral edges permanently being side sealed together, usually by soldering or welding. The open ends of this formed cylindrical container are then necked in and flanged. A bottom end closure is then double seamed thereto, and after product filling, a top end closure is similarly applied.
For the various reasons discussed above, it can be seen that a tinplate can stock which has improved formability and improved corrosion resistance is desirable. U.S. Pat. No. 4,157,694 to Nemoto et al., discloses a method of increasing the formability of tinplate having an iron-tin alloy layer. That reference teaches that an alloy layer formed by flow melting produces a hard and brittle layer. Nemoto et al. also proposes that a very fine structured iron-tin alloy layer of a thickness of 0.005 to 0.2 g./m..sup.2 first be formed electrochemically on the steel substrate and thereafter electroplating a layer of tin over the electrochemically formed tin layer. A potential problem is foreseen with this method in that it represents a significant departure from tinplate stock manufacturing resulting in increased capital expenditure, and possible reluctance by tinplate producers to commercially adopt such a method; therefore, an alternative method is desired.